The present invention relates to compositions and processes for producing borohydride compounds. In particular, the present invention provides efficient processes and compositions for the large-scale production of borohydride compounds.
Environmentally friendly fuels (e.g., alternative fuels to hydrocarbon based energy sources) are currently of great interest. One such fuel is borohydride, which can be used directly as an anodic fuel in a fuel cell or as a hydrogen storage medium (e.g., hydrogen can be liberated by the reaction of sodium borohydride with water, which produces sodium borate as a byproduct). As with all fuels, borohydride must be manufactured from readily available materials. Thus, there is a need for improved and energy efficient industrial scale manufacturing processes for producing borohydride compounds.
Typical industrial processes for the production of sodium borohydride are based on the Schlesinger process (Equation 1) or the Bayer process (Equation 2), which are both described below. Equation 1 illustrates the reaction of alkali metal hydrides with boric oxide, B2O3, or trimethoxyborate, B(OCH3)3, at high temperatures (e.g., ca. 330 to 350xc2x0 C. for B2O3 and 275xc2x0 C. for B(OCH3)3). These reactions, however, provide poor molar economy by requiring four moles of sodium to produce one mole of sodium borohydride.
4NaH+B(OCH3)3xe2x86x923NaOCH3+NaBH4xe2x80x83xe2x80x83(1)
Na2B4O7+16Na+8H2+7SiO2xe2x86x924NaBH4+7Na2SiO3xe2x80x83xe2x80x83(2)
The primary energy cost of these processes stems from the requirement for a large excess of sodium metal (e.g., 4 moles of sodium per mole of sodium borohydride produced). Sodium metal is commercially produced by electrolysis of sodium chloride with an energy input equivalent to about 17,566 BTU (18,528 KJ) per pound of sodium borohydride produced. In contrast, the hydrogen energy stored in borohydride is about 10,752 BTU (11,341 KJ) of hydrogen per pound of sodium borohydride. The Schlesinger process and the Bayer process, therefore, do not provide a favorable energy balance, because the energy cost of using such large amounts of sodium in these reactions is high compared to the energy available from sodium borohydride as a fuel.
Furthermore, in view of the large quantities of borohydride needed for use e.g., in the transportation industry, these processes would also produce large quantities of NaOCH3 or Na2SiO3 waste products. Since these byproducts are not reclaimed or reused, further energy and/or expense would need to be expended to separate and dispose of these materials.
Typical improvements of the prior art describe simple modifications of the two processes given in equations 1 and 2. Accordingly, such improvements also suffer from the disadvantages stated above, and do not provide any improved energy efficiency. Furthermore, with the widespread adoption of borohydride as a source of hydrogen, a recycle process that allows regeneration of borohydride from borate is attractive. Thus, borohydride can be used as a fuel, and the resulting borate can then be recycled back to generate borohydride. Such a process cannot rely on the same sodium stoichiometry shown in the current borohydride manufacture processes, e.g., the Schlesinger process of Equation 1 or the Bayer process of Equation 2.
The present invention provides processes for producing large quantities of borohydride compounds, which overcome these deficiencies. In addition, the efficiencies of the processes of the present invention can be greatly enhanced over the typical processes for producing borohydride compounds.
In one embodiment of the present invention, a process is provided for producing borohydride compounds, which includes: (A) reacting methane with a Y-containing species of formula Y2O to obtain Y, carbon monoxide and H2; (B) reacting the Y with H2 to obtain YH; (C) reacting a boron-containing species of the formula BX3 with the YH to obtain YHBX3; (D) separately reacting BX3 with H2 to obtain B2H6 and HX; and (E) reacting the YHBX3 with B2H6 to obtain YBH4 and BX3. Y is selected from the group consisting of the alkali metals, pseudo-alkali metals, an ammonium ion, and quaternary amines of formula NR4xe2x88x92, wherein R is independently selected from H and straight or branched C1 to C4 alkyl groups; and X is selected from the group consisting of halides, alcohols, alkoxides, chalcogens, and chalcogenides.
In another embodiment of the present invention, a process is provided for producing borohydride compounds, which includes: (A) reacting a boron-containing species of the formula BX3 with H2 to obtain B2H6 and HX; and (B) reacting the B2M6 with a Y-containing species of the formula Y2O to obtain a YBH4 and a YBO2. Y and X are the same as defined above.
In either of these embodiments, the Y-containing species of the formula Y2O and the boron-containing species of the formula BX3 can be obtained by the following two processes. The first process includes: (I) reacting a borate of the formula YBO2 with HX to obtain YX, BX3, and water; (2) reacting the YX with water to obtain YOH and HX; and converting the YOH to Y2O and H2O. The second process includes: (i) reacting a borate of the formula YBO2 with CO2 and H2O to obtain YHCO3 and B2O3; (ii) converting the YHCO3 to Y2O, CO2, and H2O; and (iii) reacting the B2O3 with HX to obtain BX3 and H2O.
In still another embodiment of the present invention, a process is provided for producing borohydride compounds, which includes: (A) reacting a borate of the formula YBO2 with CO2 and H2O to obtain YHCO3 and B2O3; (B) converting the YHCO3 to Y2O, CO2, and H2O; (C) reacting the B2O3 with C and X2 to obtain BX3 and CO2; (D) reacting methane with Y2O to obtain Y, carbon monoxide and H2; (E) reacting the Y with H2 to obtain YH; (F) reacting the BX3 with the YH to obtain YHBX3; (G) separately reacting BX3 with H2 to obtain B2H6 and HX; and (H) reacting the YHBX3 with B2H6 to obtain YBH4 and BX3. Y and X are the same as defined above.
In still another embodiment of the present invention, the process described in the previous embodiment is altered by replacing steps (D) to (H) with the following steps (D2) and (E2): (D2) reacting the BX3 with H2 to obtain B2H6 and HX; and (E2) reacting B2H6 with the Y2O to obtain a YBH4 and a YBO2.